RICHMOND, Va. — Mitt Romney, struggling to remain the frontrunner in a turbulent GOP presidential primary field, won a major backer Friday in Virginia Gov. Bob McDonnell, a rising star in his party.

Romney's campaign announced McDonnell's endorsement Friday morning from South Carolina, where McDonnell will join the former Massachusetts governor later in the day to campaign ahead of that state's Saturday primary.

McDonnell, who also chairs the Republican Governors Association, said in the campaign's news release that Romney has been an effective leader in business, with the Olympics and as a governor.

The endorsement of a Southern governor with solid support among social conservatives buoys Romney with Rick Santorum surging among conservatives and Texas Gov. Rick Perry announcing his support for former House Speaker Newt Gingrich.

For months, when pressed on his presidential preference, McDonnell had said only that he would prefer a governor. With the exit of Perry and former Utah Gov. Jon Huntsman, Romney is the lone candidate with such credentials.

"Marvelous record in a blue state, great private sector experience in creating jobs, and has been an optimistic, results-oriented conservative. And that's why it's time now for me to get involved," McDonnell said in a Friday morning interview on CNBC.

"It's a Southern primary. I'm a Southern governor and I thought I could help the governor by coming out today," he said.

In a conference call with reporters from New York, where the governor was pursuing an economic development venture, McDonnell said Perry's exit and new entreaties from Romney's campaign influenced the timing of his announcement.

"My friendship with Governor Perry was important. Certainly, with a more narrow field, (it) makes the choices even that much more clear," he said. "I've been looking at this for a number of months and have come to the decision over time. This was just the best time to announce it publicly."

He said he had called Perry to tell him of his decision before announcing it.

McDonnell's backing gives Romney an important strategic boost in Virginia, where Romney is in a one-on-one contest with Texas Rep. Ron Paul. They were the only Republicans to qualify for the ballot in the March 6 primary.

Romney already had the backing of the state's Republican lieutenant governor, Bill Bolling, who chairs his campaign in Virginia.